A conventional rotation detector has been disclosed in Japanese Unexamined Patent Publication No. JP-2-264817. When such a rotation detector is used as a rotation detector for a two-cycle vehicle (for example, motorcycle), a magnetic conversion element (Hall IC) mounted to a circuit board and a magnet are arranged in a resin case in a substantially cylindrical shape having a detecting face for detecting an object for detection along the detecting face, and the circuit board, the magnetic conversion element and the magnet are sealed by a sealing member comprising epoxy etc. thereby constituting a rotation detector. Further, it is general to install the rotation detector to a transmission case or a sprocket cover for detecting a blade edge etc. of a transmission gear or a sprocket rotating at the inside thereof as the object for detection.
When a tooth, etc. of a transmission gear or a sprocket is detected as the object for detection by using such a rotation detector, more or less clearance is present at the object for detection and therefore, a vibration is caused in the object for detection depending on the rigidity of the body of a vehicle (two-cycle vehicle) or an amount of exhaust etc. whereby a gap between the object for detection and a detecting position of the rotation detector is varied, causing a change in magnetism in the rotation detector. The change produces a noise in detection which gives rise to cause erroneous operation of, for example, an indicating instrument displaying the speed in accordance with output data outputted from the rotation detector.
Further, the shape and the size of the object for detection are varied according to the kind of vehicle and therefore, it is necessary to adjust the gap for detection between the object for detection and the rotation detector which complicates the integrating operation performance.